You should only connect one sensor per zone unless they are none powered devices such as door contacts panic buttons etc. If you connect two sensors to one zone and have a false alarm it becomes difficult to tell which detector activated. Its less so with non powered devices as they normally either work or dont work. If you have a false alarm from a non powered device you can and it still works you can normally tell which one is failing by its resistance.
Using EOL doubles the devices lowers the amount of cores needed, it also means that the panel can tell you high resistance tamper faults when there are problems with the cable. It can be fiddly fitting EOL resistors in the detectors (some come with EOL's onboard and you just use the dipswitch's for the right values)
Most detectors have a 15 metre range if you have rooms bigger than this and want to use two motion detectors in the same room, try to make sure the coverage does not overlap. If overlapping coverage is unavoidable then use two different protocols such as a PIR and a Dual Tech. This is a requirement for monitored systems but it is also good practice for all systems. After a false alarm people tend to look at the log and see the last thing that activated and not get the full story from the log, it just means that they end up with several false alarms before the cause is found.
Its great you son is involved in this, one of my NSI inspectors was on the Autistic spectrum, a bit of a strange character, but very very thorough and if something wasnt right he would find it.